Corn stover fuel objects with high heat output and reduced emissions designed for large-scale power generation

ABSTRACT

A Novel fuel object comprised of a proportion of corn stover and a proportion of wood fiber combined with a basically reacting compound. The object comprises fiber of the appropriate size and moisture content combined with an inorganic base. An appropriately sized object is readily manufactured, provides high heat output, is consistent in fuel characteristics, and is sized and configured for use in power generation facilities. Based on fiber selection and processing, the fuel object may be used in a variety of current power generation technologies including stoker, fluidized bed, gasifier, cyclonic, direct-fired, and pulverized coal technologies, and results in significant reduction of air emissions (including sulfur dioxide, nitrogen oxides, hydrochloric acid, carbon monoxide, carbon dioxide, and mercury) compared to coal with no loss of boiler or furnace efficiency.

CROSS REFERENCES TO RELATED APPLICATIONS

This application claims priority to and the benefit as a continuationapplication of U.S. patent application entitled, “Corn Stover FuelObjects With High Heat Output And Reduced Emissions Designed ForLarge-Scale Power Generation,” Ser. No. 13/422,863, filed Mar. 16, 2012,which is a continuation application of Ser. No. 13/074,256, filed Mar.29, 2011, which is a continuation application of Ser. No. 12/359,766,filed Jan. 26, 2009, the entire contents of which are incorporatedherein by reference and relied upon.

BACKGROUND

The invention relates a high-energy, low-emission solid fuel made fromnatural renewable feedstocks.

In large part, energy generation in the United States and worldwide isbased on combustion of conventional non-renewable fossil fuels such ascoal, coal byproducts, petroleum-based oils and natural gas products.These energy sources provide a source of energy for power plants,industrial facilities, and institutions, however, they are not renewablein nature, have significant environmental impacts, are decreasing insupply, and increasing in cost. Continued use of these fossil fuelenergy sources results in cumulative environmental impact includingincreased local and global concentrations of greenhouse gases, sulfurdioxide, nitrogen oxides, and mercury.

The decreasing supply and environmental impacts associated with fossilfuel use have led to consideration of potential for energy derived fromcombustion of natural products, typically cellulosic materials. Many ofthe natural sources comprising potential cellulosic fuels have notachieved commercial success in the past due to a variety of problemsincluding high moisture content, low fuel heating value, impurities inthe fuel, inconsistent fuel characteristics, transportation costs,difficulties in handling, and high processing costs. Further, suchmaterials often have combustion problems or compositions that result inthe formation of adverse emissions and substantial quantities of ash.

We have found, in the energy market segment, a substantial need for anew cellulosic based fuel containing substantially no conventional BTUsource from fossil fuels such as coal, petroleum, natural gas or othersuch non-renewable sources. This need relates to a fuel that has asubstantial heating value, is consistent in nature, is low in moisture,can be readily made at low cost, can be transported and handled at lowcost, can be used in existing solid fuel systems with little or nomodifications, has lower emissions than fossil fuels, and isspecifically adapted for use in modern power plant installations.

SUMMARY

We have now found a fuel source that provides a substantial heat outputsatisfactory for use in large-scale power generation, even in theabsence of coal, oil, gas or other conventional fossil fuels. A formedcylindrical object, briquette or cube is comprised of a blend ofcellulosic material including corn stover and wood. As used herein, theterm “cube”, “briquette,” “formed object,” or “fuel object” are roughlysynonymous and refer to a discrete particle of any size or shape thatcontains the natural cellulosic materials described herein. The majordimension of the fuel object is less than about 6 cm. The volume of thefuel object is about 10 to 100 cm3. “Conventional fossil fuel” refers tocoal products including bituminous coal, anthracite coal, peat, coke andcoking byproducts and to petroleum products such as oil, gas, naturalgas liquids and products derived from shale and tar sands.

We have found that moisture content and particle size of the corn stoverand wood fiber particles in the final fuel object are important forproduct formation, handling, and effective combustion. We have foundthat the addition of an effective amount of base material reducescorrosive and acidic byproducts from the combustion of the cellulosicmaterials and reduces emissions of sulfur oxides, nitrogen oxides,hydrogen chloride and other acidic materials. The processing and blendof materials provides a high energy output without the addition of anyfossil fuels such as coal, oil or natural gas as found in prior artmaterials. In the preferred materials of the invention, no separatelyadded binder is used.

We have also found that the fuel object of the invention can be madewithout conventional binder materials such as that used in forming anumber of the prior art materials. Such binders, in the prior art, aretypically polymeric binders or are additional lignent or hemicellulosesmaterials.

We have found that the fuel object, based on sizing and specifications,may be used immediately in existing solid fuel energy facilities(including those employing stoker, fluidized bed, gasifier, cyclonic,direct-fired, and pulverized coal technologies) and operate with anefficiency and higher heating heat value similar to some coals with asignificant reduction in air emissions per million BTU of energy outputcompared to fossil fuels.

Additional features and advantages are described herein, and will beapparent from the following Detailed Description and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows the cooperation in the reduction in formation of SO₂ inemissions from the use of base in combustion of the fuel of theinvention.

FIG. 2 shows the cooperation in the reduction in formation of NO_(x) inemissions from the use of base in combustion of the fuel of theinvention.

FIG. 3 shows the reduction in formation of HCl in emissions from thecombustion of the fuel of the invention.

FIG. 4 shows the reduction in formation of both SO₂ and NO_(x) inemissions from the combustion of the fuel of the invention.

FIG. 5 is a process diagram for the fuel object forming process.

FIG. 6 shows an embodiment of the fuel object of the invention.

DETAILED DESCRIPTION

The embodiment that is the subject of this application includes acylindrical object, briquette, cube or other such formed objectcomprising corn stover, wood and an inorganic base. The inventioninvolves a fuel object processed, sized and configured for use inlarge-scale modern combustion energy generating systems. Composition,raw material particle size, moisture content, and fuel object size andstructure, result in the fuel object having a high energy value, lowemissions upon combustion, and highly efficient combustion, in a rangeof commercially available combustion technologies.

The fuel object can have a volume of about 10 to 100 cm³; does not needto be symmetrical, but it is preferred that the fuel object besubstantially symmetrical in shape such as cylindrical object, a cube,parallel-piped or the like. A cube can be roughly a rectangular prism ora cube that is 1 to 6 cm on a side. Typical density of the fuel objectis 30 to 40 lbs/fl³. A cylindrical object can be about 0.5 to 10 cm inheight and about 0.5 to 10 cm in diameter.

The fuel object typically comprises about 60 to 80 weight percent cornstover and 20 to 40 weight percent wood. The corn stover component has aparticle size of about 100 to 35,000 microns with about 90 percent ofthe particles greater than 1,000 microns. The wood fiber has a particlesize of about 100 to 30,000 microns with about 90 percent of theparticles greater than 1,000 microns.

The initial reduction of the particle size of the corn stover and woodfiber, to these proportions before fuel object formation, provides aninput to the process that forms the fuel object leading to amechanically stable fuel object that can be manufactured, stored,transported and used in modern combustion installations as is or iseasily comminuted to a small particle size depending on the nature ofthe combustion process.

Chemical bases that can be used in a fuel object of the inventioninclude typically alkali metal and alkaline earth metal bases. Suchbases can be made from sodium potassium, calcium, magnesium and othersuch metal species. The base can be used in the form of oxides,hydroxides, carbonates, bicarbonates, phosphates, and any otherinorganic anion that produces a basically reacting solution, a pHgreater than 7.5, when the base is mixed in water (pH about neutral) atan amount of about 0.5 to 10%. The important characteristic of thechemical base is that during the combustion process the chemical basecan react with combustion byproducts such as sulfur oxides (SO₂),nitrogen oxides (NO_(X)), chloride (Cl⁻¹) hydrochloric acid (HCl) andother acidic producing gaseous species. The chemical base can aid informing neutralized species and substantially reduced the effects ofcorrosive action on combustion equipment. Specifically, corn stover hashigher chlorine content than wood and use of the base additive helpsreduce chlorine/chloride gas formation upon combustion of the fuelobject. A preferred base to neutralize acids in combustion is abicarbonate or dolomite.

The fuel objects have a typical heating value of at least about 7,000BTU/lb (about 3,500 cal.-gm⁻¹); about 7,300 BTU/lb (about 3,650cal.-gm⁻¹) and typically at least 8,000 BTU/lb (about 4000 cal.-gm⁻¹)all on a dry basis.

FIG. 5 is a diagram of the process for the manufacture of the fuelobject of the invention. The process 500 includes, as primary processstations, raw materials and fuels storage 501, raw materials dryer unit503, a cooling unit 504 and a cubing or object forming unit 505. Theseprimary stations take blended wood fiber and renewable fuel component,adjust particle size, reduce moisture to a preferred level, cool thematerial and then form the fuel object as needed. Once formed, the fuelis then stored or transferred to an industrial combustion or burn unit(not shown).

The raw materials used in making the object of the invention isdelivered to and stored in raw material delivery and storage unit 501.That material is then transferred to a pretreatment screen 511 for thepurpose of separating fines from useful material. Useful material isthen transferred to a pretreatment hammer mill which adjusts the fibersize to appropriate fiber dimensions for final object formation. Finematerials from both the prescreening, pretreatment and from thehammermill are transferred to the colt exhaust bag house 515 through theconduit A-A. The sized fuel is transferred to a feed bin 512 fortemporary storage. The sized fuel source is then transferred using adryer and feed conveyor 513 to the dryer drum 503 for drying purposes.The output from dryer 503, having moisture content of about 0.1 to 14wt. % moisture, is then directed to drop out chamber 514(a) thatseparates fines from the appropriately sized materials in a dried form.Heat for the dryer 503 is generated by Burner 506 and blending chamber507. Fuel for the burner 506 is stored in bin 510. Fuel is transferredto the burner 506 through transfer line B-B from bin 510. Heat andrecycle gas stream from dryer 514 b is sent conduit 508 through chamber507 to the dryer 503. Heat is recycled to the burner 506 throughconveyor 509.

The fines are directed to cyclone dryer 514(b) while the appropriatelysized materials are directed to cooling drum in feed conveyor 517.Exhaust from the cyclone material is directed to recycle fan 522 whichdirects the exhaust either to ambient air or to the recycle throughblending chamber 507. The cold exhaust bag house 515 takes fines fromthe screen and hammermill station 511 and from the cubing stations 505.Those bag house fines are collected on the particulate conveyor 516which are combined from the 10 output from the cyclone dryer 514(b) andare directed to the pulling drum and feed conveyor 517. Cooling drum 504cools the particulate material from the infeedment conveyor 517 and thenconveys that material on conveyor 519 to the fuel object formationstations 505. Optionally, if the cooling drum is not required forprocessing the fiber, the fiber before entry into the cooling drum inconveyor 517 can be directed to a dryer bypass bulk conveyor 518 andthen to an optional dryer cooling drum bypass feed conveyor 523 thatdirects the fiber to the object formation stations 505. Fines fromscreen 511, through airlock 521, the object forming units 505 and fromdrum 504 or conveyor 517 are directed to baghouse 515. Air is vented toambient from baghouse 515 from vent 520. The input material placed indelivery and storage unit 501 is typically preblended with theappropriate amount of wood fiber and renewable fuel source. Once formed,the fuel objects of the invention can be stored in product stock pile502 and then transferred to a combustion unit (not shown) for energygeneration).

Reduced Air Emissions

The composition of the fuel and chemical base additive are designed tominimize air emissions from combustion of the fuel object. The impact ofthe addition of inorganic base additives at different concentrations onreduction of sulfur dioxide, nitrogen oxide and hydrochloric acidemissions during combustion of the fuel object was evaluated inbench-scale testing. The testing, is summarized in FIGS. 1-3 below,support that use of the additive results in reduced emissions ofhydrochloric acid, sulfur dioxide and nitrogen oxide emissions.

The objects contain less than 0.5 percent sulfur by weight percent andtherefore emit approximately 95 percent less sulfur dioxide emissionsthan derivation of a similar amount of energy from coal. Because woodand corn stover are biogenic in nature, combustion of the fuel object isconsidered carbon neutral under carbon registries and trading programsin place in the United States today and therefore results in a 100percent reduction in creditable greenhouse gas emissions than derivationof a similar amount of energy from coal. Nitrogen content of the cornstover and wood and addition of the inorganic base additive also resultin an approximately 40 percent reduction of nitrogen oxide emissionsthan derivation of a similar amount of energy from coal. FIG. 4 belowdemonstrates the reduction in tested emission rates of nitrogen oxideand sulfur dioxide where eastern coal is replaced 100 percent with thefuel object.

Data collected from combustion of the objects in a 15 percent fuelobject/85 percent coal co-fire with eastern coal also confirms emissionreductions of sulfur dioxide and carbon monoxide when compared to 100percent eastern coal. Table 1 demonstrates the reductions in sulfurdioxide and carbon monoxide where the fuel object replaced 15 percenteastern coal in a 170,000 lb/hour steam boiler compared to combustion of100 percent eastern coal.

TABLE Emission Reductions from 15 Percent Replacement of Easter Coalwith Claimed Fuel Gaseous Pollutant Emissions (lb/MMBtu) Test ID FuelSO₂ CO₂ Baseline 1 2.49 0.081 Baseline 2 100% Coal 2.28 0.083 Baseline 32.48 0.085 Baseline 4 2.63 0.102 Cofire 1 Blended Fuel 2.12 0.089 Cofire2 (85.1:14.9 - coal:Fuel) 2.11 0.081 Cofire 3 (14.9% wood) 2.26 0.081Baseline Averages 2.47 ± 0.14 0.088 ± 0.010 Cofire Averages 2.16 ± 0.080.083 ± 0.05  % Difference −12.4% −5.02%

Object and Particle Size

The size and density of the fuel object is significant in that it allowsfor ease of handling, transportation, storage and conveyance in mostpower generation facilities. Object size also is important in that itallows the fuel object to burn on the grate of stoker-type combustionunits and not combust prematurely.

Particle size within the fuel object also is significant in bothmanufacturing a fuel object that maintains its integrity though shippingand handling and that burns efficiently. Efficient combustion reducesemissions of nitrogen oxides and carbon monoxide and leaves minimalresidue, such as ash, which would have to be disposed in a waste site.Sizing of the fuel particles also is critical to allow a fuel object tobreak into discrete particles in certain applications such as pulverizedcoal-type units.

Combustion Efficiency

The blend, composition, moisture and size of the fuel object allowefficient operation in existing power generation facilities. Testing bythe Southern Research Institute demonstrated that use of the fuel objectin combination with coal in a large steam boiler resulted in no loss ofboiler efficiency compared to 100 percent coal. Table 2 provides asummary of collected data.

TABLE 2 Boiler Efficiency Comparison of 100 Percent Coal to 15 PercentFuel Object and 85 Percent Coal (Source SRI April 2008) Heat Input HeatOutput Efficiency Test ID Fuel (MMBtu/hr) (MMBtu/hr) (%) Baseline 1 100%Coal 264.6 224.4 84.8 Baseline 2 100% Coal 264.3 223.9 84.8 Baseline 3100% Coal 264.8 223.7 84.5 Baseline 4 100% Coal 267.6 228.8 84.5 Cofire1 85.1 Coal/14.9 275.7 229.7 83.3 Fuel Object Cofire 2 85.1 Coal/14.9271.9 230.0 84.6 Fuel Object Cofire 3 85.1 Coal/14.9 272.5 230.3 84.5Fuel Object Baseline NA 265.3 225.2 84.9 ± 0.4 Average Cofire Average NA273.4 230.0 84.1 + 0.7 Absolute NA 8.1 4.8 −0.7 Difference % DifferenceNA 3.0% 2.1% −0.9% Statistically NA NA NA NA Significant Change?

Corn Stover

Corn, also known as maize (Zea mays; also ssp. mays L.), is the sourceof corn stover. The term “corn stover” refers to the remains from thesurface component of a corn plant, aside from the corn kernels. Stoverdoes not typically include large amounts of the crown or surface roots.Corn stover is generally the leaves, stalk, husk, etc. Corn stover canalso include incidental grasses and weeds. Corn stover is one of thelargest sources of low cost cellulose in the United States. About oneton of corn stover is produced for every one ton of corn grain. About250 million dry tons of stover are produced each year. Thus, the UnitedStates produces substantial amounts of corn stover for potential use infuel applications.

Wood Fiber

The term “wood fiber” refers to a product derived from some part of atree as that term is commonly used in the art. A number of directproducts and byproducts can be derived by taking trees or portions oftrees and reducing their particle size. The term “wood fiber” may referto materials derived from fruit, leaves, sap, bark and other such treebyproducts. Wood fiber is typically derived from either the woody partof the tree within the bark and typically refers to either wood-likecomponents of tree trunks, tree limbs and tree roots. Wood fiber istypically primarily cellulosic in nature but is known to be derived fromwood cells that typically comprise a substantial proportion ofcellulosic materials in combination with lignin and hemicellulosicmaterials in a fibrous woody cell structure. Wood fiber can be derivedfrom a number of tree sources including both hard and soft woods. Suchwood fiber materials can be derived from the processing of trees intosized lumber, the byproduct of clearing and shredding trees, thebyproducts derived from any process that begins with a wood containingplant part leading to the formation of a substantially cellulosic woodfiber material.

General Method of Manufacturing a Fuel Object

The process for manufacturing fuel objects described herein starts bygrinding cellulosic material. The cellulosic material can be ground byfeeding a pulverizer or grinder to reduce the cellulosic material to apredetermined size. The corn stover component is ground to a size ofabout 100 to 35,000 microns with about 90 percent of the particlesgreater than 1,000 microns. The wood fiber is ground to a particle sizeof about 100 to 30,000 microns with about 90 percent of the particlesgreater than 1,000 microns).

After grinding, cellulosic materials a may be fed through a dryer. Adryer ensures that moisture content of the cellulosic material is atless than about 14 wt. % and often less than 10 wt. %. Preferably,moisture of a finished fuel object should be between 7 wt. % to 14 wt.%. Moisture content of the cellulosic material is significant to theintegrity of a fuel object since moisture content of cellulosic materialassists in bonding all of the materials in the composition prior to andfollowing the pelletizing of the composition. However, an increase inthe moisture of the cellulosic material beyond a disclosed limit wouldjeopardize the characteristics of the fuel object and its ability towithstand being transported. It is important that an object maintain itsintegrity prior to precombustion processing or burning. A fuel objectshould be rigid enough to be handled mechanically without crumbling.Achieving proper and desired moisture content for a fuel object iscritical to achieve a desired heat output and in maintaining the abilityto transport fuel objects without harming its integrity, shape, orcomposition.

After the cellulosic material has been sufficiently dried to desiredmoisture content, the cellulosic material can be fed through a secondarypulverizer as necessary. A secondary pulverizer can be the finalgrinding process for the cellulosic materials.

The base additive may be added once organic components have beenappropriately sized and conditioned. Components of a fuel object may befurther blended together by means of a blender, drum or other mechanicalequipment.

A densification process can create a final composition of materials.Densification allows materials to be mixed and blended in a controlledmanner with other particles comprising a fuel object. After thecomponents of a fuel object have been sufficiently blended, thecomponents/materials are processed and forced through densificationequipment. Such equipment could include commercially available machinessuch as those produced by Warren and Baerg. Densification equipmentforces a blended composition through a shaping die, thereby creating thefuel object. Faceplate temperature of the object forming extrusionequipment typically is between 165° F. and 185° F. A fuel object exitsat a temperature of about 110° F. and not greater than 145° F. When afuel object exits the extruder, there can be a slight coating on theexternal surface of the object. This coating can comprise lignin, whichis a naturally occurring substance of the cellulosic material. Objectsare then transferred to the finished object conveyor/cooler.

Following formation, fuel objects are cooled down by a cooling meansincluding, but not limited to, an air cooler, an air conditioner, orliquid nitrogen. The cooling process causes the objects to harden intothe shape created by the extruder and allows components of the object tomaintain their integrity. In an embodiment, fuel objects are placedthrough a shaker screen after sufficiently cooling and hardening. Thisprocess separates fine and discrete particles of the composition. Thedischarge for the fine particles can be separated from a fuel object andare again recycled or forced through a extruder. This process minimizesthe potential for waste generated by any excess particles that comprisea fuel object. The final object maintains its structure and provides anease of handling and a highly consistent product for good combustion atthe end user power facility.

Use Integration at Power Facilities

The fuel object is designed for immediate use in existing solid fuelfired systems. This may include facilities that produce heat or steamfor cooling, heating, or electrical generation or direct induration ordrying of a product. Such facilities may include power plants,industrial furnaces and boilers and steam and power generationfacilities at large institutions such as universities and hospitals. Theformed and densified fuel object is ideal for transport by truck, rail,and conveyer and storage in bunkers and silos that are designed fortransport and storage of coal. In most instances the objects may beunloaded, stored, and transported at a facility by the existingmechanisms that transport coal without physical modification.

The fuel object is ideal for use in stoker fired systems where fuel isdischarged on a large grate and burns on the grate over a period oftime. The fuel object also works in systems where fuel is pulverizedprior to entry of the combustion chamber and then combusted insuspension such as pulverized coal, cyclonic combustion, anddirect-fired units. Because individual particles in the fuel object arereduced in size prior to cubing and dried to a low moisture content,they combust efficiently and with low emissions when fed through suchsuspension-based systems and do not result in slagging or increased ashor sparklers from unburned fuel. Because the fuels have such a higheating value, burn efficiently, and reduce emissions, facilities thatuse the fuel object may reduce emissions without capital expenditure onemission controls and maintain unit efficiencies.

Example Corn Stover Fuel Object

A fuel object derived from corn stover and wood was produced and wassubstantially free of coal or other fossil fuel source. FIG. 6 shows anembodiment of the fuel object of the invention. The corn stover fuelobject was substantially cylindrical in shape with a length of 6.0 cmand a diameter of 2.5 cm.

TABLE 2 Fuel Objection Characterization Volume 15 cm³ Moisture content9.58 wt. % Amount of particulate derived from corn 64.2 wt. %stover-fiber size 100 to 35,00 microns Amount of particulate derivedfrom wood 26.1 wt. % fiber-fiber size 100 to 30,000 microns Sodiumbicarbonate (inorganic base)  9.7 wt. %

In testing, this corn stover fuel object provided about 7,422 BTU/pound(lb.).

Although the invention has been disclosed in the context of certainembodiments and examples, it will be understood by those skilled in theart that the invention extends beyond the specifically disclosedembodiments to other alternative embodiments and/or uses and obviousmodifications and equivalents thereof. Accordingly, the invention is notintended to be limited by the specific disclosures of preferredembodiments herein, but instead by reference to claims attached hereto.Reference to a single element in the claims is intended not exclude oneor more of the same element. The above specification, examples and dataprovide a complete description of the manufacture and use of thecomposition of the invention. The invention resides in the claimshereinafter appended.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present subjectmatter and without diminishing its intended advantages. It is thereforeintended that such changes and modifications be covered by the appendedclaims.

The invention is claimed as follows:
 1. A fuel object having maximumdimension of about 6 cm, a volume of about 10 to 100 cm³, a moisturecontent of less than about 14 wt. %, the fuel object comprising: (i)about 60 to 80 wt. % of a particulate derived from corn stover having aparticle size of about 100 to 35,000 microns; (ii) about 20 to 40 wt. %of a wood fiber having a particle size of about 100 to 30,000 microns;and (iii) about 1 to 10 wt. % of sodium bicarbonate; and (iv) whereinthe fuel object is substantially free of coal as a heat source and thefuel object provides at least 7,000 BTU-lb⁻¹ (3500 cal.-gm⁻¹).
 2. Thefuel object of claim 1 wherein the fuel object comprises a cubic unitwith a side dimension of 2 to 6 cm.
 3. A fuel source comprising a fuelobject according to claim 1 and a conventional fossil fuel.
 4. The fuelsource of claim 3 wherein the conventional fossil fuel comprises coal.5. A method of reducing emissions upon combustion of a conventionalfossil fuel, said method comprising co-firing a fuel object of claim 1with a conventional fossil fuel.
 6. The method of claim 5 wherein HCl,SO₂ and NO_(x) emissions are reduced.
 7. The method of claim 5 whereinthe conventional fossil fuel is selected from coal products andpetroleum products.
 8. The method of claim 7 wherein the coal productsare selected from bituminous coal, anthracite coal and peat.
 9. Themethod of claim 8 wherein the coal comprises eastern coal.